Lubricant composition

ABSTRACT

The present invention relates to a lubricating oil composition containing a mineral oil (A) containing 30% by mass or more of a bright stock (A1) on the basis of the total amount of the lubricating oil composition, wherein the lubricating oil composition further contains a synthetic oil (B) and a phosphoric acid ester (C), the content of the synthetic oil (B) is less than 70% by mass on the basis of the total amount of the lubricating oil composition, the content of the phosphoric acid ester (C) as expressed in terms of a phosphorus atom is 0.050 to 0.200% by mass on the basis of the total amount of the lubricating oil composition, and a viscosity index is 150 or more, thereby providing a lubricating oil composition having low temperature dependence and favorable lubricating characteristics even in the case of using the mineral oil containing a bright stock.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition.

BACKGROUND ART

As a rotating part of a tool machine, in order to attain an improvement in processing efficiency, one capable of rotating at a high speed even when a high load is applied is preferred. In general, grease is frequently used for a lubricating oil of rolling bearings.

However, when a rotational speed of spindle becomes high, in the grease, its supply becomes insufficient, so that employment of oil-air lubrication becomes mainstream.

In the oil-air lubrication that is used for lubrication of a rotating part of a tool machine, the lubricating oil that is intermittently supplied from a mixing valve is discharged inside of the pipe and then attached in a form of finely divided oil droplets onto the surface of the inside of the pipe owing to a flow of compressed air to be continuously supplied inside of the pipe.

Here, the lubricating oil that is used for oil-air lubrication is required to have such properties that not only is it possible to form a firm oil film on the surface of the inside of the pipe, but also a change in viscosity owing to the temperature of the inside of the pipe is small.

For example, PTL 1 discloses a lubricating oil composition to be used for oil-air lubrication, which contains not only 20 to 80% by mass of an ester oil but also a poly-α-olefin or an ether oil.

CITATION LIST Patent Literature

PTL 1: JP 2014-84429 A

SUMMARY OF INVENTION Technical Problem

Now, as for the lubricating oil composition, there may also be a requirement from the standpoint of economy, such as cost reduction, in addition to lubricating characteristics. For example, the lubricating oil composition disclosed in PTL 1 uses synthetic oils with a high price, such as an ester oil, a poly-α-olefin, and an ether oil and is required to attain an improvement from the viewpoint of cost reduction.

From the viewpoint of cost reduction, in addition to use of a mineral oil in place of the synthetic oil, among mineral oils, it is required to utilize a bright stock as a high-viscosity mineral oil, which is obtained by subjecting a residual oil after vacuum distillation of a crude oil to a deasphalting treatment, a dewaxing treatment, a hydrorefining treatment, and the like.

In the case of using such a mineral oil containing a bright stock, though there is superiority from the standpoint of cost reduction, there is involved such a problem that it is inferior to a lubricating oil composition using a synthetic oil from the standpoint of temperature dependence and lubricating characteristics.

Therefore, even in the case of using a mineral oil containing a bright stock, it is required to develop a lubricating oil composition having characteristics equal to those in the case of using a synthetic oil.

An object of the present invention is to provide a lubricating oil composition having low temperature dependence and favorable lubricating characteristics even in the case of using a mineral oil containing a bright stock.

Solution to Problem

The present inventor has found that in a lubricating oil composition containing 30% by mass or more of a mineral oil containing a bright stock, when the contents of a synthetic oil and a phosphoric acid ester are adjusted to predetermined ranges, the aforementioned problem can be solved, thereby leading to accomplishment of the present invention.

Specifically, the present invention provides the following [1] to [12].

[1] A lubricating oil composition containing 30% by mass or more of a mineral oil (A) containing a bright stock (A1) on the basis of the total amount of the lubricating oil composition, wherein

the lubricating oil composition further contains a synthetic oil (B) and a phosphoric acid ester (C),

the content of the synthetic oil (B) is less than 70% by mass on the basis of the total amount of the lubricating oil composition,

the content of the phosphoric acid ester (C) as expressed in terms of a phosphorus atom is 0.050 to 0.200% by mass on the basis of the total amount of the lubricating oil composition, and

a viscosity index is 150 or more.

[2] The lubricating oil composition as set forth in [1], wherein a kinematic viscosity at 100° C. of the bright stock (A1) is 28.0 mm²/s or more. [3] The lubricating oil composition as set forth in [1] or [2], wherein a content proportion of the bright stock (A1) in the mineral oil (A) is 5 to 70% by mass on the basis of the total amount of the mineral oil (A). [4] The lubricating oil composition as set forth in any of [1] to [3], wherein the content of the synthetic oil (B) is 10% by mass or more and less than 70% by mass on the basis of the total amount of the lubricating oil composition. [5] The lubricating oil composition as set forth in any of [1] to [4], wherein a viscosity index of the synthetic oil (B) is 130 or more. [6] The lubricating oil composition as set forth in any of [1] to [5], wherein a kinematic viscosity at 100° C. of the synthetic oil (B) is 80 to 180 mm²/s. [7] The lubricating oil composition as set forth in any of [1] to [6], wherein the synthetic oil (B) contains a poly-α-olefin (B1). [8] The lubricating oil composition as set forth in any of [1] to [7], wherein the phosphoric acid ester (C) contains a compound (C1) selected from an acidic phosphoric acid ester and an amine salt of an acidic phosphoric acid ester. [9] The lubricating oil composition as set forth in any of [1] to [8], wherein the phosphoric acid ester (C) contains a compound (C1) selected from an acidic phosphoric acid ester and an amine salt of an acidic phosphoric acid ester and a neutral phosphoric acid ester (C2). [10] The lubricating oil composition as set forth in [9], wherein a content ratio [(C2)/(C1)] of the neutral phosphoric acid ester (C2) to the compound (C1) is 2.0 to 40 in terms of a mass ratio. [11] The lubricating oil composition as set forth in any of [1] to [10], further containing a sulfur atom-containing extreme pressure agent,

the content of the sulfur atom-containing extreme pressure agent as expressed in terms of a sulfur atom being less than 0.12% by mass on the basis of the total amount of the lubricating oil composition.

[12] The lubricating oil composition as set forth in any of [1] to [11], which is used for an oil-air lubrication system.

Advantageous Effects of Invention

The lubricating oil composition of the present invention uses a mineral oil containing a bright stock and not only has superiority from the standpoint of cost reduction but also has low temperature dependence and favorable lubricating characteristics.

DESCRIPTION OF EMBODIMENTS [Lubricating Oil Composition]

The lubricating oil composition of the present invention is one containing a mineral oil (A) containing a bright stock (A1), a synthetic oil (B), and a phosphoric acid ester (C), which is adjusted so as to have a viscosity index of 150 or more.

When the viscosity index is less than 150, for example, in the case of being used for the oil-air lubrication, there is a case where it becomes difficult to form a firm oil film on the surface of the inside of a pipe depending upon the temperature of the inside of the pipe.

In order to avoid the aforementioned evil, though the viscosity index of the lubricating oil composition of the present invention is 150 or more, it is preferably 151 or more, and more preferably 153 or more.

The kinematic viscosity at 100° C. of the lubricating oil composition as one embodiment of the present invention is preferably 30.0 to 60.0 mm²/s, more preferably 35.0 to 55.0 mm²/s, and still more preferably 40.0 to 52.0 mm²/s.

In this specification, the kinematic viscosity and the viscosity index mean values measured and calculated in conformity with JIS K2283:2000.

It is preferred that the lubricating oil composition as one embodiment of the present invention further contains an antioxidant (D).

In addition, the lubricating oil composition as one embodiment of the present invention may further contain an additive for lubricating oil other than the components (C) and (D) within a range where the effects of the present invention are not impaired.

In the lubricating oil composition as one embodiment of the present invention, the total content of the component (A), the component (B), and the component (C) is preferably 70 to 100% by mass, more preferably 75 to 100% by mass, still more preferably 80 to 100% by mass, and yet still more preferably 90 to 100% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition.

In addition, in the lubricating oil composition as one embodiment of the present invention, the total content of the component (A) and the component (B) is preferably 65% by mass or more, more preferably 71% by mass or more, still more preferably 76% by mass or more, yet still more preferably 81% by mass or more, and even yet still more preferably 86% or more, and it is preferably 99.9% by mass or less, more preferably 99.5% by mass or less, and still more preferably 99.0% by mass or less, on the basis of the total amount (100% by mass) of the lubricating oil composition.

Each of the components that are contained in the lubricating oil composition of the present invention is hereunder described in detail.

<Mineral Oil (A)>

Examples of the mineral oil (A) that is contained in the lubricating oil composition of the present invention include atmospheric residual oils obtained by subjecting a crude oil, such as a paraffinic crude oil, an intermediate base crude oil, and a naphthenic crude oil, to atmospheric distillation; distillates obtained by subjecting such an atmospheric residual oil to vacuum distillation; mineral oils obtained by subjecting the foregoing distillate to at least one treatment, such as solvent deasphalting, solvent extraction, hydrogenation, solvent dewaxing, catalytic dewaxing, and hydrorefining; and mineral oils (GTL) obtained by isomerizing a wax (GTL wax (gas-to-liquids WAX) produced from a natural gas by the Fischer-Tropsch process or the like.

The mineral oil (A) may be used alone or may be used in combination of two or more thereof.

In the lubricating oil composition of the present invention, from the viewpoint of cost reduction, the content of the mineral oil (A) is 30% by mass or more on the basis of the total amount (100% by mass) of the lubricating oil composition; however, from the viewpoint of securing the blending amounts of the synthetic oil (B) and the phosphoric acid ester (C) and providing a lubricating oil composition having low temperature dependence and favorable lubricating characteristics, it is preferably 30 to 90% by mass, more preferably 30 to 80% by mass, still more preferably 30 to 70% by mass, and yet still more preferably 30 to 60% by mass.

From the viewpoint of cost reduction, the mineral oil (A) that is contained in the lubricating oil composition of the present invention contains a bright stock (A1).

The “bright stock” as referred to in this specification means a high-viscosity mineral oil produced by subjecting a residual oil after vacuum distillation of a crude oil, such as a paraffinic crude oil, an intermediate base crude oil, and a naphthenic crude oil, to a treatment selected from solvent deasphalting, solvent extraction, solvent dewaxing, and hydrorefining, and so on.

The kinematic viscosity at 100° C. of the bright stock (A1) is preferably 28.0 mm²/s or more, more preferably 29.0 mm²/s or more, and still more preferably 30.0 mm²/s or more, and it is typically 40.0 mm²/s or less, and preferably 35.0 mm²/s or less.

From the viewpoint of providing a lubricating oil composition having low temperature dependence, the viscosity index of the bright stock (A1) is preferably 80 or more, more preferably 90 or more, and still more preferably 100 or more.

The paraffin content (% C_(P)) of the bright stock (A1) that is used in one embodiment of the present invention is preferably 50 to 90, more preferably 60 to 85, and still more preferably 70 to 80.

The naphthene content (% C_(N)) of the bright stock (A1) is preferably 10 to 40, more preferably 15 to 35, and still more preferably 20 to 30.

The aromatic content (% C_(A)) of the bright stock (A1) is preferably 0 to 30, more preferably 0 to 20, and still more preferably 0 to 10.

In this specification, the values of the paraffin content (% C_(P)), the naphthene content (% C_(N)), and the aromatic content (% C_(A)) are proportions (percentages) of the paraffin content, the naphthene content, and the aromatic content, respectively as measured according to ASTM D-3238 ring analysis (n-d-m method).

From the viewpoint of cost reduction and the viewpoint of providing a lubricating oil composition having low temperature dependence and favorable lubricating characteristics, a content proportion of the bright stock (A1) in the mineral oil (A) is preferably 5 to 70% by mass, more preferably 10 to 67% by mass, still more preferably 12 to 65% by mass, and yet still more preferably 14 to 62% by mass on the basis of the total amount (100% by mass) of the mineral oil (A).

From the viewpoint of adjusting to a predetermined viscosity, it is preferred that the mineral oil (A) that is used in one embodiment of the present invention contains a mineral oil (A2) other than the bright stock (A1).

As the mineral oil (A2), the mineral oils mentioned above are exemplified, and a mineral oil classified into any of Groups 1 to 3 in the base oil category by the American Petroleum Institute (API) and a mineral oil obtained by isomerizing the GTL wax can be used; however, from the viewpoint of cost reduction, mineral oils classified into Group 1 or 2 are preferred.

In the lubricating oil composition of the present invention, even by using a mineral oil classified into Group 1 or 2, a lubricating oil composition having low temperature dependence and favorable lubricating characteristics may be prepared.

The kinematic viscosity at 100° C. of the mineral oil (A2) is preferably 7.0 to 15.0 mm²/s, more preferably 9.0 to 13.0 mm²/s, and still more preferably 10.0 to 12.0 mm²/s.

The viscosity index of the mineral oil (A2) is preferably 80 or more, more preferably 90 or more, and still more preferably 100 or more.

The paraffin content (% C_(P)) of the mineral oil (A2) that is used in one embodiment of the present invention is preferably 50 to 90, more preferably 60 to 85, and still more preferably 70 to 80.

The naphthene content (% C_(N)) of the mineral oil (A2) is preferably 10 to 40, more preferably 15 to 35, and still more preferably 20 to 30.

The aromatic content (% C_(A)) of the mineral oil (A2) is preferably 0 to 20, more preferably 0 to 10, and still more preferably 0 to 5.

<Synthetic Oil (B)>

Examples of the synthetic oil (B) that is contained in the lubricating oil composition of the present invention include poly-α-olefins; isoparaffins; various esters, such as polyol esters and dibasic acid esters; various ethers, such as polyphenyl ethers; polyalkylene glycols; alkylbenzenes; and alkylnaphthalenes.

The synthetic oil (B) may be used alone or may be used in combination of two or more thereof.

In the lubricating oil composition of the present invention, from the viewpoint of cost reduction, the content of the synthetic oil (B) is less than 70% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition; however, from the viewpoint of providing a lubricating oil composition having low temperature dependence and favorable lubricating oil characteristics, it is preferably 10% by mass or more and less than 70% by mass, more preferably 20% by mass or more and less than 70% by mass, still more preferably 30% by mass or more and less than 70% by mass, and yet still more preferably 40% by mass or more and less than 65% by mass.

The kinematic viscosity at 100° C. of the synthetic oil (B) that is used in one embodiment of the present invention is preferably 80 to 180 mm²/s, more preferably 85 to 175 mm²/s, and still more preferably 90 to 170 mm²/s.

The viscosity index of the synthetic oil (B) is preferably 130 or more, more preferably 150 or more, and still more preferably 170 or more.

It is preferred that the synthetic oil (B) that is used in one embodiment of the present invention contains a poly-α-olefin (B1) from the viewpoint of providing a lubricating oil composition having low temperature dependence and favorable lubricating characteristics even in the case of using the mineral oil containing a bright stock.

The poly-α-olefin (B) may be an α-olefin homopolymer or may be an α-olefin copolymer.

Examples of the α-olefin copolymer include copolymers of ethylene and an α-olefin having 8 to 20 carbon atoms (preferably 8 to 14 carbon atoms).

The poly-α-olefin (B1) may be used alone or may be used in combination of two or more thereof.

Even in the case of using the mineral oil containing a bright stock, from the viewpoint of providing having low temperature dependence and favorable lubricating characteristics, the poly-αolefin (B1) is preferably a poly-α-olefin obtained through polymerization of an α-olefin as the raw material by using a metallocene catalyst (the poly-α-olefin will be hereinafter also referred to as “m-PAO”).

A content proportion of the poly-α-olefin (B1) in the synthetic oil (B) is preferably 50 to 100% by mass, more preferably 65 to 100% by mass, still more preferably 80 to 100% by mass, and yet still more preferably 90 to 100% by mass on the basis of the total amount (100% by mass) of the synthetic oil (B).

<Phosphoric Acid Ester (C)>

Even in the case of using the mineral oil containing a bright stock, from the viewpoint of providing a lubricating oil composition having improved lubricating characteristics, in particular, improved wear resistance and heat resistance, the lubricating oil composition of the present invention preferably contains a phosphoric acid ester (C).

Examples of the phosphoric acid ester (C) include neutral phosphoric acid esters, such as an aryl phosphate, an alkyl phosphate, an alkenyl phosphate, and an alkylaryl phosphate; acidic phosphoric acid esters, such as a monoaryl acid phosphate, a diaryl acid phosphate, a monoalkyl acid phosphate, a dialkyl acid phosphate, a monoalkenyl acid phosphate, and a dialkenyl acid phosphate; phosphorous acid esters, such as an aryl hydrogen phosphite, an alkyl hydrogen phosphite, an aryl phosphite, an alkyl phosphite, an alkenyl phosphite, an arylalkyl phosphite; and acidic phosphorous acid esters, such as a monoalkyl acid phosphite, a dialkyl acid phosphite, a monoalkenyl acid phosphite, and a dialkenyl acid phosphate.

The acidic phosphoric acid ester and the acidic phosphorous acid ester may be each an amine salt.

The phosphoric acid ester (C) may be used alone or may be used in combination of two or more thereof.

In the lubricating oil composition of the present invention, from the viewpoint of an improvement in lubricating characteristics, in particular, improvements in wear resistance and heat resistance, though the content of the phosphoric acid ester (C) as expressed in terms of a phosphorus atom is 0.050 to 0.200% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition, it is preferably 0.055 to 0.150% by mass, more preferably 0.060 to 0.110% by mass, and still more preferably 0.065 to 0.100% by mass.

In this specification, the content as expressed in terms of a phosphorus atom means a value measured in conformity with JPI-5S-38-03.

In the lubricating oil composition as one embodiment of the present invention, in particular, from the viewpoint of more improving the heat stability to provide a lubricating oil composition capable of maintaining the excellent wear resistance even in the case of continuing the use in a high-temperature environment, the phosphoric acid ester (C) preferably contains a compound (C1) selected from an acidic phosphoric acid ester and an amine salt of an acidic phosphoric acid ester, and more preferably contains the compound (C1) and a neutral phosphoric acid ester (C2).

In the lubricating oil composition as one embodiment of the present invention, from the aforementioned viewpoint, the content of the compound (C1) is preferably 0.010 to 0.50% by mass, more preferably 0.015 to 0.40% by mass, still more preferably 0.020 to 0.30% by mass, and yet still more preferably 0.025 to 0.25% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition.

In one embodiment of the present invention, in the case where the phosphoric acid ester (C) contains the compound (C1) and the neutral phosphoric acid ester (C2), a content ratio [(C2)/(C1)] of the neutral phosphoric acid ester (C2) to the compound (C1) is preferably 2.0 to 40, more preferably 2.2 to 32, still more preferably 2.4 to 25, and yet still more preferably 2.5 to 15 in terms of a mass ratio.

As the acidic phosphoric acid ester that may be selected as the compound (C1), a compound represented by the following general formula (c1) is preferred.

In the general formula (c1), n is 1 or 2; and R^(a)'s are each independently an alkyl group having 1 to 18 carbon atoms (preferably 1 to 15 carbon atoms).

In the case where n is 2, then the plural R^(a)'s may be the same as or different from each other.

As the amine salt of an acidic phosphoric acid ester that may be selected as the compound (C1), an amine salt of the compound represented by the general formula (c1) is preferred. The amine constituting the foregoing amine salt is preferably a compound represented by the following general formula (c-i).

(R^(c))_(q)—N—(H)_(3-q)  (c-i)

In the general formula (c-i), q represents an integer of 1 to 3.

R^(c)'s are each an alkyl group having 6 to 18 carbon atoms, an alkenyl group having 6 to 18 carbon atoms, an aryl group having 6 to 18 ring-forming carbon atoms, an arylalkyl group having 7 to 18 carbon atoms, or a hydroxyalkyl group having 6 to 18 carbon atoms.

In the case where a plurality of R^(c)'s are present, the plural R^(c)'s may be the same as or different from each other.

The neutral phosphoric acid ester (C2) is preferably a compound represented by the following general formula (c2-1), and more preferably a compound represented by the following general formula (c2-2).

In the general formula (c2-1), R¹ to R³ are each independently an alkyl group having 1 to 12 carbon atoms; or an aryl group having 6 to 18 ring-forming carbon atoms, which is substituted with an alkyl group having 1 to 12 carbon atoms.

In the general formula (c2-2), R¹¹ to R¹³ are each independently an alkyl group having 1 to 12 carbon atoms, and preferably an alkyl group having 1 to 4 carbon atoms.

p1 to p3 are each independently an integer of 1 to 5, preferably an integer of 1 to 2, and more preferably 1.

<Antioxidant (D)>

It is preferred that the lubricating oil composition as one embodiment of the present invention further contains an antioxidant (D).

In the lubricating oil composition as one embodiment of the present invention, the content of the antioxidant (D) is preferably 0.1 to 7.0% by mass, more preferably 0.2 to 5.0% by mass, and still more preferably 0.3 to 3.0% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition.

In one embodiment of the present invention, the antioxidant (D) preferably contains at least one selected from a phenol-based antioxidant (D1) and an amine-based antioxidant (D2), and more preferably contains both a phenol-based antioxidant (D1) and an amine-based antioxidant (D2).

In one embodiment of the present invention, a content ratio [(D1)/(D2)] of the phenol-based antioxidant (D1) to the amine-based antioxidant (D2) is preferably 1/5 to 5/1, more preferably 1/4 to 4/1, and still more preferably 1/3 to 3/1 in terms of a mass ratio.

Examples of the phenol-based antioxidant (D1) include monophenol-based compounds, such as 2,6-di-t-butylphenol, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, isooctyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate; diphenol-based compounds, such as 4,4′-methylenebis(2,6-di-t-butylphenol) and 2,2′-methylenebis(4-ethyl-6-t-butylphenol); and hindered phenol-based compounds.

Examples of the amine-based antioxidant (D2) include diphenylamine compounds, such as an alkylated diphenylamine having an alkyl group having 3 to 20 carbon atoms; and naphthylamine compounds, such as α-naphthylamine, phenyl-α-naphthylamine, and a substituted phenyl-α-naphthylamine having an alkyl group having 3 to 20 carbon atoms.

<Additive for Lubricating Oil>

The lubricating oil composition as one embodiment of the present invention may contain an additive for lubricating oil other than the aforementioned components (C) and (D) within a range where the effects of the present invention are not impaired.

Examples of such an additive for lubricating oil composition include a viscosity index improver, a pour-point depressant, a detergent, a dispersant, a rust inhibitor, a metal deactivator, a demulsifier, and an anti-foaming agent.

Each of these additives for lubricating oil may be used alone or may be used in combination of two or more thereof.

Although the content of each of these additives for lubricating oil can be appropriately adjusted within a range where the effects of the present invention are not impaired, it is typically 0.001 to 10% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition.

In this specification, taking into consideration handling properties and solubility in the mineral oil (A), the additive, such as a viscosity index improver and an anti-foaming agent, may be blended in a form of a solution thereof having been diluted in a part of the mineral oil (A), with the other components.

In such a case, in this specification, the content of the additive, such as an anti-foaming agent and a viscosity index improver, means the content as expressed in terms of the active components exclusive of a diluting oil (expressed in terms of the resin content).

In the lubricating oil composition as one embodiment of the present invention, an extreme pressure agent other than the component (C) may be contained within a range where effects of the present invention are not impaired; however, it is preferred that the content of the extreme pressure agent is as small as possible, and in particular, it is preferred that the content of a sulfur atom-containing extreme pressure agent is as small as possible.

Examples of the sulfur atom-containing extreme pressure agent include sulfur-based extreme pressure agents, such as dibenzyl disulfide; and sulfur-phosphorus-based extreme pressure agents, such as O,O,O-triphenylphosphorothioate.

In the lubricating oil composition as one embodiment of the present invention, the content of the sulfur atom-containing extreme pressure agent as expressed in terms of a sulfur atom is preferably less than 0.12% by mass, more preferably less than 0.07% by mass, and still more preferably less than 0.01% by mass on the basis of the total amount (100% by mass) of the lubricating oil composition.

In this specification, the content as expressed in terms of a sulfur atom means a value measured in conformity with JIS K2541-6:2013.

[Characteristics and Use of Lubricating Oil Composition]

The lubricating oil composition of the present invention uses the mineral oil containing a bright stock and has not only superiority from the standpoint of cost reduction but also low temperature dependence and favorable lubricating characteristics, in particular, wear resistance.

On the occasion of subjecting the lubricating oil composition as one embodiment of the present invention to a four-ball test under a condition at an oil temperature of 60° C. and at a number of rotation of 1,200 rpm under a load of 392 N for a test time of 60 minutes in conformity with ASTM D4172, a wear track diameter of a stationary ball after the test (wear track diameter by four-ball test before heat stability test) is preferably 0.55 mm or less, more preferably 0.50 mm or less, and still more preferably 0.47 mm or less.

In addition, on the occasion of subjecting the lubricating oil composition as one embodiment of the present invention to a heat stability test at 150° C. for 168 hours in conformity with JIS K2540 and then performing a four-ball test under a condition at an oil temperature of 60° C. and at a number of rotation of 1,200 rpm under a load of 392 N for a test time of 60 minutes in conformity with ASTM D4172, a wear track diameter of a stationary ball after the test (wear track diameter by four-ball test after heat stability test) is preferably 0.70 mm or less, more preferably 0.65 mm or less, and still more preferably 0.60 mm or less.

Although the lubricating oil composition of the present invention can be applied to a variety of lubricating uses, it is preferably used for an oil-air lubrication system because it has low temperature dependence and favorable lubricating characteristics, such as excellent wear resistance.

That is, the present invention is also able to provide (1) an oil-air lubrication system as mentioned below and (2) a use method of a lubricating oil composition as mentioned below.

Suitable embodiments of the lubricating oil composition as prescribed in the following (1) and (2) are those mentioned above.

(1) An oil-air lubrication system for supplying a lubricating oil composition inside of a pipe by compressed air,

the lubricating oil composition containing 30% by mass or more of a mineral oil (A) containing a bright stock (A1), and

further containing a synthetic oil (B) and a phosphoric acid ester (C), wherein

the content of the synthetic oil (B) is less than 70% by mass,

the content of the phosphoric acid ester (C) as expressed in terms of a phosphorus atom is 0.050 to 0.200% by mass, and

a viscosity index is 150 or more.

(2) A use method of a lubricating oil composition to be used for an oil-air lubrication system,

the lubricating oil composition containing 30% by mass or more of a mineral oil (A) containing a bright stock (A1), and

further containing a synthetic oil (B) and a phosphoric acid ester (C), wherein

the content of the synthetic oil (B) is less than 70% by mass,

the content of the phosphoric acid ester (C) as expressed in terms of a phosphorus atom is 0.050 to 0.200% by mass, and

a viscosity index is 150 or more.

EXAMPLES

The present invention is hereunder described in detail by reference to Examples, but it should be construed that the present invention is by no means limited by these Examples. Various properties of respective components used in the Examples and Comparative Examples and obtained lubricating oil compositions were measured in conformity with the following methods.

<Kinematic Viscosity and Viscosity Index>

Measurement and calculation were performed in conformity with JIS K2283:2000.

<Paraffin Content (% C_(P)), Naphthene Content (% C_(N)), and Aromatic Content (% C_(A))>

Measurement was performed according to ASTM D-3238 ring analysis (n-d-m method).

<Content of Phosphorus Atom>

Measurement was performed in conformity with JPI-5S-38-03.

<Content of Sulfur Atom>

Measurement was performed in conformity with JIS K2541-6:2013.

Examples 1 to 7 and Comparative Examples 1 to 2

A mineral oil, a synthetic oil, and various additives as mentioned below were added in blending amounts shown in Table 1 and thoroughly mixed, to prepare lubricating oil compositions, respectively.

Details of the mineral oils and the various additives used in the Examples and Comparative Examples are those mentioned below.

<Mineral Oil>

“Bright Stock”:

A bright stock classified into Group II in the API category, kinematic viscosity at 100° C.=30.9 mm²/s, viscosity index=107, % C_(P)=73, % C_(N)=27, % C_(A)=0.

“500 N Mineral Oil”:

A mineral oil classified into Group II in the API category, kinematic viscosity at 100° C.=10.9 mm²/s, viscosity index=107, % C_(P)=72, % C_(N) 28, % C_(A)=0.

<Synthetic Oil>

“m-PAO”

A poly-α-olefin obtained by using a metallocene catalyst, kinematic viscosity at 100° C.=147.4 mm²/s, viscosity index=202.

“PAO”

A poly-α-olefin, kinematic viscosity at 100° C.=100 mm²/s, viscosity index=170.

<Phosphoric Acid Ester>

“Amine Salt of Acidic Phosphoric Acid Ester (1)”:

An amine salt of an acidic phosphoric acid ester (1) represented by the foregoing general formula (c1) wherein R^(a) is a methyl group, and n is 1 or 2.

“Amine Salt of Acidic Phosphoric Acid Ester (2)”:

An amine salt of an acidic phosphoric acid ester (2) represented by the foregoing general formula (c1) wherein R^(a) is an isotridecyl group (alkyl group of C13), and n is 1 or 2.

“Acidic Phosphoric Acid Ester (3)”:

An acidic phosphoric acid ester (3) represented by the foregoing general formula (c1) wherein R^(a) is an octadecyl group (alkyl group of C18), and n is 1 or 2.

“Neutral Phosphoric Acid Ester (1)”:

Tricresyl phosphate that is a compound represented by the foregoing general formula (c2-2) wherein R¹¹ to R¹³ are each a methyl group, and p1 to p3 are each 1.

“Neutral Phosphoric Acid Ester (2)”:

Mono- or di-t-butyltriphenyl phosphate that is a compound represented by the foregoing general formula (c2-2) wherein R¹¹ to R¹³ are each a t-butyl group, and p1 to p3 are each 1 or 2.

<Extreme Pressure Agent Other than Phosphorus-Based Extreme Pressure Agent>

“Sulfur-Based Extreme Pressure Agent”:

Dibenzyl disulfide

“Sulfur-Phosphorus-Based Extreme Pressure Agent”:

O,O,O-Triphenylphosphorothioate, phosphorus atom content=8.9% by mass.

<Antioxidant>

“Phenol-Based Antioxidant”:

Octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate

“Amine-Based Antioxidant”:

Monobutylphenyl monooctylphenylamine

<Other Additives>

“Pour-Point Depressant”:

Polymethacrylate

“Metal Deactivator”:

Benzotriazole

“Rust Inhibitor”:

Ester of alkenylsuccinic acid and polyhydric alcohol

“Anti-Foaming Agent”:

Silicone-based anti-foaming agent

With respect to each of the lubricating oil compositions prepared in the Examples and Comparative Examples, the kinematic viscosity at 100° C. and the viscosity index were those shown in Table 1.

Each of the lubricating oil compositions was used as sample oil and measured for the wear track diameter by the four-ball test before and after the heat stability test in conformity with the following methods. These measurement results are also shown in Table 1.

[Wear Track Diameter by Four-Ball Test Before Heat Stability Test]

The four-ball test was performed under a condition at an oil temperature of 60° C. and at a number of rotation of 1,200 rpm under a load of 392 N for a test time of 60 minutes in conformity with ASTM D4172, and the wear track diameter (mm) of a stationary ball after the test was measured, a value of which was defined as the “wear track diameter by four-ball test before heat stability test”.

[Wear Track Diameter by Four-Ball Test after Heat Stability Test]

The sample oil was subjected to the heat stability test at 150° C. for 168 hours in conformity with JIS K2540.

Then, the sample oil after the heat stability test was subjected to the four-ball test under a condition at an oil temperature of 60° C. and at a number of rotation of 1,200 rpm under a load of 392 N for a test time of 60 minutes in conformity with ASTM D4172, and the wear track diameter (mm) of a stationary ball after the test was measured, a value of which was defined as the “wear track diameter by four-ball test after heat stability test”.

TABLE 1 Exam- Exam- Exam- Exam- Exam- Exam- Exam- Comparative Comparative ple 1 ple 2 ple 3 ple 4 ple 5 ple 6 ple 7 Example 1 Example 2 Composi- Mineral Bright stock mass 30.00 30.00 30.00 30.00 30.00 6.00 10.00 44.70 26.10 tion oil % 500 N mass 22.36 22.05 22.27 21.84 21.99 34.05 26.79 18.11 26.71 mineral oil % Synthetic m-PAO mass 45.00 45.00 45.00 45.00 45.00 35.00 45.00 oil % PAO mass 57.00 60.26 % Phosphoric Amine salt mass 0.03 acid ester of acidic % phosphoric acid ester (1) Amine salt mass 0.34 0.34 0.34 of acidic % phosphoric acid ester (2) Acidic mass 0.12 phosphoric % acid ester (3) Neutral mass 0.90 0.90 0.90 0.90 0.90 0.90 phosphoric % acid ester (1) Neutral mass 1.00 0.53 0.53 phosphoric % acid ester (2) Extreme Sulfur-based mass 0.40 pressure extreme % agent other pressure than agent phosphorus- Sulfur- mass 0.45 0.45 0.45 based phosphorus- % extreme based pressure extreme agent pressure agent Antioxidant Phenol-based mass 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 antioxidant % Amine-based mass 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 antioxidant % Other Pour-point mass 0.50 0.50 0.50 0.50 0.50 0.50 0.50 additive depressant % Metal mass 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 deactivator % Rust mass 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 inhibitor % Anti- mass 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 foaming % agent — Total mass 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 % Content proportion of bright stock mass 57.30 57.64 57.39 57.87 57.70 14.98 27.18 71.17 49.42 in mineral oil (100% by mass) % Content of phosphoric acid ester as mass 0.078 0.085 0.078 0.075 0.075 0.085 0.085 0.040 0.040 expressed in terms of a phosphorus % atom Content of sulfur atom-containing mass — — — 0.05 0.10 — — 0.05 0.05 extreme pressure agent as expressed % in terms of a sulfur atom Content ratio of (neutral phosphoric — 30.00 2.65 7.50 — — 2.65 2.65 — — acid ester)/(acidic phosphoric acid ester) Properties Kinematic viscosity at mm²/s 45.7 45.6 45.6 45.8 45.7 44.7 49.7 40.1 43.3 100° C. Viscosity index — 153 153 153 154 153 151 152 142 153 Test Wear track diameter by mm 0.32 0.44 0.51 0.50 0.49 0.46 0.43 0.57 0.57 four-ball test before heat stability test Wear track diameter by mm 0.58 0.47 0.51 0.65 0.64 0.48 0.46 Not Not four-ball test after measured measured heat stability test

It is noted from Table 1 that the lubricating oil compositions of Examples 1 to 7 have a high viscosity index and low temperature dependence, and as compared with those of Comparative Examples 1 to 2, are small in the wear track diameter by four-ball test before heat stability test and excellent in the wear resistance.

In Comparative Examples 1 to 2, the value of the wear track diameter by four-ball test before heat stability test was large, resulting in inferior wear resistance. Thus, the measurement regarding the value of the “wear track diameter by four-ball test after heat stability test” was not performed, and the test was finished. 

1. A lubricating oil composition comprising 30% by mass or more of a mineral oil (A) containing a bright stock (A1) on the basis of the total amount of the lubricating oil composition, wherein the lubricating oil composition further comprises a synthetic oil (B) and a phosphoric acid ester (C), the content of the synthetic oil (B) is less than 70% by mass on the basis of the total amount of the lubricating oil composition, the content of the phosphoric acid ester (C) as expressed in terms of a phosphorus atom is 0.050 to 0.200% by mass on the basis of the total amount of the lubricating oil composition, and a viscosity index is 150 or more.
 2. The lubricating oil composition according to claim 1, wherein a kinematic viscosity at 100° C. of the bright stock (A1) is 28.0 mm²/s or more.
 3. The lubricating oil composition according to claim 1, wherein a content proportion of the bright stock (A1) in the mineral oil (A) is 5 to 70% by mass on the basis of the total amount of the mineral oil (A).
 4. The lubricating oil composition according to claim 1, wherein the content of the synthetic oil (B) is 10% by mass or more and less than 70% by mass on the basis of the total amount of the lubricating oil composition.
 5. The lubricating oil composition according to claim 1, wherein a viscosity index of the synthetic oil (B) is 130 or more.
 6. The lubricating oil composition according to claim 1, wherein a kinematic viscosity at 100° C. of the synthetic oil (B) is 80 to 180 mm²/s.
 7. The lubricating oil composition according to claim 1, wherein the synthetic oil (B) contains a poly-α-olefin (B1).
 8. The lubricating oil composition according to claim 1, wherein the phosphoric acid ester (C) contains a compound (C1) selected from an acidic phosphoric acid ester and an amine salt of an acidic phosphoric acid ester.
 9. The lubricating oil composition according to claim 1, wherein the phosphoric acid ester (C) contains a compound (C1) selected from an acidic phosphoric acid ester and an amine salt of an acidic phosphoric acid ester and a neutral phosphoric acid ester (C2).
 10. The lubricating oil composition according to claim 9, wherein a content ratio [(C2)/(C1)] of the neutral phosphoric acid ester (C2) to the compound (C1) is 2.0 to 40 in terms of a mass ratio.
 11. The lubricating oil composition according to claim 1, further comprising a sulfur atom-containing extreme pressure agent, the content of the sulfur atom-containing extreme pressure agent as expressed in terms of a sulfur atom being less than 0.12% by mass on the basis of the total amount of the lubricating oil composition.
 12. The lubricating oil composition according to claim 1, which is used for an oil-air lubrication system. 